1. Field of the Invention
This invention relates to a vibration damping system for a vehicle for damping vibration of a particular vibrating element such as the vehicle body, air in the cabin or the like mainly generated by vibration of a power unit, and more particularly to a vibration damping system for a vehicle which has a vibrator for vibrating the particular vibrating element and vibrates the particular vibrating element in the phase reverse to that of the vibration of the particular vibrating element and in the amplitude equal to that of the vibration of the same, thereby damping the vibration of the vehicle body or air in the cabin (noise).
2. Description of the Prior Art
As disclosed in Japanese Unexamined Patent Publication No. 1(1989)-501344, there has been known a vibration damping system for a vehicle comprising, in addition to the vibrator, a vibration sensor which detects vibration of a particular vibrating element on the vehicle body and a drive control means which performs calculation on the basis of the detecting signal from the vibration sensor and causes the vibrator to vibrate the particular vibrating element so that the vibration of the particular vibrating element is damped. In such vibration damping systems, there are those in which an optimization technique is employed in the calculation performed by the drive control means as in the vibration damping system disclosed in the above identified patent publication and there are those in which the optimization technique is not employed. The former systems are generally arranged as shown in FIG. 33.
The vibration damping system shown in FIG. 33 is for damping vibration of air in the cabin (noise) generated due to vibration of an engine E (as the power unit) and comprises a plurality of microphones 2 (m in number) which are disposed in predetermined positions in the cabin and detect vibration of air in the respective predetermined positions, a plurality of speakers 4 (i in number) which are disposed in predetermined positions in the cabin and vibrate air in the cabin to damp the vibration of air, and a drive control means 6 which generates drive signals y.sub.1 to y.sub.i for controlling the speakers 4. The vibration damping system is further provided with a reference signal generating means 8 which detects an ignition pulse signal w, generated in relation to the revolution speed of the engine E, from an ignition coil 24 and shapes the waveform of the ignition pulse signal w, thereby generating a reference signal x.
The microphones 2 detect the vibration due to the vibration of the engine E together with the vibration generated by the speakers 4 and output the result of the detection as detecting signals e.sub.1 to e.sub.m. The detecting signals e.sub.1 to e.sub.m are input into the drive control means 6 through amplifiers 16 and A/D convertors 18. The reference signal x generated by the reference signal generating means 8 is input into the drive control means 6 through an amplifier 12 and an A/D convertor 14.
The drive control means 6 comprises adaptive filters F1 to Fi which adjust the phase and the amplitude of the reference signal x and an adaptive algorithm section 10 which updates every moment the factors of the adaptive filters F1 to Fi so that the detecting signals e.sub.1 to e.sub.m input from the microphones 2 are minimized, and outputs the signals passing through the adaptive filters F1 to Fi as the drive signals y.sub.1 to y.sub.i. As the adaptive algorithm for updating the factors of the adaptive filters F1 to Fi, there have been known Least Mean Square Method, Newton Method, Simplex Method, Powell Method and the like. In this example, Least Mean Square Method is employed. In Least Mean Square Method, the reference signal x is input into the adaptive algorithm section 10 through a digital filter H.degree..sub.IM (I standing for 1, 2, . . . , i and M standing for 1, 2, . . . , m). The digital filter H.degree..sub.IM is modeled on the transmission properties between I-th speaker 4 and M-th microphone 2 and the space distance between the speaker 4 and the microphone 2 is thus interpolated.
A spectral analysis shows that vibration of a power unit such as an engine includes many sinusoidal vibration components having frequencies of integral multiples of the engine rpm and that the vibration components have different levels and one or more particular components have especially high levels. By damping the vibration caused by the components of the vibration of the power unit at such high levels, a sufficient vibration damping effect can be obtained, and accordingly, control is generally effected with the aim of damping the vibration caused by the components of the vibration of the power unit at such high levels. For example, in the case of a vehicle having a four-cycle four-cylinder engine, the vibration component having a frequency of twice the engine speed (will be referred to as "the secondary component", hereinbelow) has an especially high level, and control is generally effected with intention of damping the vibration caused by the secondary component.
Though the vibration damping system described above is arranged to damp the vibration of air in the cabin (noise) caused by the engine vibration, solid elements of the vehicle body such as a frame of the vehicle body, panels of the vehicle body, seats, a steering wheel and the like are also caused to vibrate by the engine vibration. Accordingly it is preferred that not only the vibration of air in the cabin but also the vibration of the solid elements be damped. However the vibration damping system having speakers and microphones respectively as the vibrators and the vibration sensors cannot damp the vibration of the solid elements though can damp the vibration of air.
On the other hand, in the case of vibration damping system disclosed, for instance, in Japanese Unexamined Patent Publication No. 3(1991)-219139 having an engine mount which supports the engine relative to the vehicle body and also functions as a vibrator for vibrating engine (will be referred to as "the vibrating engine mount", hereinbelow) and an acceleration sensor which functions as the vibration sensor cannot damp the vibration of air with a high efficiency though can damp the vibration of the solid elements with a high efficiency.
It may be possible to satisfactorily damp both the vibration of air and the vibration of the solid elements by providing both a speaker and a vibrating engine mount and controlling them.
However when the number of the kinds of vibrators is simply increased, the load on the drive control means in calculation is increased in vain or electric power consumption to drive the vibrators excessively increases, which results in inefficient vibration damping effect.